Light-emitting, flat-panel display devices are used for a number of applications such as general illumination light sources, decorative light sources, and information displays. Organic light-emitting diode (OLED) display devices often form an array of differently colored, light-emitting elements that are either arranged spatially in a single layer of independently addressable light emitting elements as discussed by U.S. Pat. No. 5,294,869 issued to Tang and Littman, entitled “Organic electroluminescent multicolor image display device” or are composed of three stacked layers of independently addressable light emitting elements as has been discussed by U.S. Pat. No. 5,703,436 issued to Forrest et al., entitled “Transparent Contacts for Organic Devices”. To form large, high-resolution devices of either type presents significant manufacturing barriers.
The structure of an OLED typically comprises, in sequence, an anode, an organic electroluminescent (EL) medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al., “Organic electroluminescent diodes”, Applied Physics Letters, 51, 913 (1987), and U.S. Pat. No. 4,769,292, demonstrated highly efficient OLEDs using such a structure of organic material layers. Since then, numerous OLEDs with alternative organic material layer structures have been disclosed. For example, there are three organic material layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of doped organic thin films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly includes a host material doped with a guest material wherein the layer structures are denoted as HTL/LEL/ETL. Further, there are other organic material layer structures that contain a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or a hole-blocking layer, and/or an electron-blocking layer in the devices. These structures have further resulted in improved device performance. The term “EL unit” may be used to describe the organic material layers between and in electrical contact with a pair of electrodes, and will include at least one light-emitting layer, and more typically comprises, in sequence, a hole-transport layer, a light-emitting layer, and an electron-transport layer, denoted in brief as HTL/LEL/ETL. Devices having multiple layers of independently addressable light emitting elements will therefore have more than one EL unit stacked on top of one another wherein at least two of the EL units are positioned between separate pairs of electrodes.
The formation of a full-color display device through the spatial arrangement of different colors of emissive organic materials on a single layer of independently addressable light emitting elements requires at least three different organic materials to be deposited in a mosaic on a single layer of independently addressable light emitting elements. This mosaic must be deposited such that any one of the organic materials do not overlap a second organic material and further that each organic material appropriately overlaps electrodes that are formed to drive the organic display. Methods such as vapor deposition through a shadow mask are often used to accomplish this task. Unfortunately, accurate alignment of these precision shadow masks with the appropriate electrodes on the substrate requires a significant period of time to accomplish, slowing manufacturing. Of even greater concern is the fact that the shadow masks are typically not thermally stable and, therefore, it is difficult to maintain the exact tolerances necessary to correctly pattern the three-or-more colors of organic light-emitting materials onto a substrate. Further, the amount of thermal expansion that can occur with a shadow mask increases with increases in mask area, making this process more difficult, when forming OLED displays on large substrates. Other methods to pattern organic materials onto a substrate have been proposed and are a subject of research. However, only evaporation through a shadow mask has been successfully demonstrated in high-volume manufacturing.
To overcome this problem, organic materials may be uniformly deposited on a single layer, wherein these organic materials form an array of addressable EL units providing either a broadband emission or multiple spectral peaks. Red, green and blue color filters may then be used to filter the emission from the addressable array of EL units to form red, green, and blue subpixels. While this method has the advantage that it allows a full-color display to be made without requiring patterning of the deposited organic material, the color filters must provide a narrow pass band to form a full-color display, significantly reducing the efficiency of the display. In such a display, it is typical that less than one third of the light that is produced by the array of organic units is passed through the color filters.
It is possible to use other color filter arrangements to improve the efficiency of an OLED display with red, green and blue color filters. One particularly useful approach utilizes an unfiltered white subpixel in addition to the subpixels that are filtered using red, green, and blue color filters as described in U.S. patent application US 2004/0113875 assigned to Miller et al. and entitled “Color OLED display with improved power efficiency”. Using this approach, it has been shown that the efficiency of the display device can be doubled when compared to the use of subpixels having red, green, and blue color filters. In this display configuration nearly two-thirds of the light that is produced by the organic emitter is passed through the color filters when displaying typical images. Unfortunately, the layout of the underlying circuitry required to drive a display employing a fourth subpixel in each pixel limits the resolution of the display device.
Alternatively to devices having an array of EL units producing multiple colors of light emission within a single layer of independently addressable light emitting elements, independently addressable layers of red, green and blue light emitting elements may be formed in a passive-matrix stacked OLED display structure as described by U.S. Pat. No. 5,703,436 issued to Forrest et al., entitled “Transparent Contacts for Organic Devices”. A full color display device of this type is created by forming at least three layers of independently addressable light emitting elements as shown in FIG. 1. Referring to FIG. 1, a display device of this type is formed by forming a first electrode 12 on a substrate 10. An EL unit 14 (comprised of one or more light-emitting organic material layers, and optionally additional material layers as further discussed below) is then formed over the first electrode, followed by a second electrode 16. Successive EL units 18 and 22 and electrodes 20 and 24 are then formed over this second electrode 16. To form a full-color device in this way requires the formation of at least three EL units 14, 18, and 22 and four electrodes 12, 16, 20, and 24. This method has the advantage that it does not, necessarily, require the organic materials to be spatially patterned. That is, if three layers of independently addressable light emitting elements can be formed, one emitting red light, a second emitting green light, and a third emitting blue light, then a full-color display may be formed without spatially patterning the EL materials. The display device can also theoretically be higher in resolution since the red, green, and blue subpixels are formed in the same location within the array of light emitting elements that form the display device.
Unfortunately, a robust manufacturing process for forming display devices of this type has not been demonstrated and active-matrix structures for forming such displays have not been disclosed. Issues such as the need to connect four stacked electrodes to circuitry on a substrate; the lack of electrode materials that are substantially transparent and have a high enough work function to serve as a cathode; and the need for TFT structures that can be used and connected to the four separate electrodes all prohibit the robust manufacturing of devices having three or more layers of independently addressable light emitting elements.
Multi-color displays having multiple layers of independently addressable light emitting elements have also been discussed in the prior art that are formed having two emissive layers of independently addressable light emitting elements. U.S. Patent Application 2005/0012465 filed by Uchida and entitled “Organic electroluminescent device, method for driving the same, illumination device, and electronic apparatus” describes such an OLED device structure. In this device structure, three electrodes are formed on a substrate wherein the center electrode and one of the first and third electrodes have transparency. An EL unit is formed between each pair of electrodes. This device is driven such that by switching between a forward driving in which the first and third electrodes serve as anodes and inverse driving where the first and third electrodes serve as cathodes either the first or second EL unit is driven with a forward bias and emits light while the remaining EL unit is simultaneously driven with a reverse bias and does not emit light. By driving this display device in this way, the device may emit either the color of light produced by the first or the second EL unit. If the drive is switched from forward to inverse bias quickly enough, both EL units are perceived as emitting light and the device is perceived as emitting a mixture of the color of light produced by the first and second EL unit. Such a device structure does not require the first and third electrodes to be driven independently of one another, potentially reducing the circuitry required to drive the display device. This patent application does not, however, describe a full-color display device or provide an active matrix means for driving the display device.
Full color displays having two layers of independently addressable light emitting elements have also been disclosed. U.S. Pat. No. 6,747,618 issued to Arnold et al. and entitled “Color organic light emitting diode display with improved lifetime” discusses a display having a layer of independently addressable blue light emitting elements above which are patterned an array of red and green EL units to form a second layer of independently addressable light emitting elements capable of providing red and green light. Such a display device provides a longer lifetime since the blue light emitting OLEDs generally have a shorter lifetime than red or green light emitting OLEDs and this structure allows the area of blue light emitting diodes to be increased, decreasing the current density required to drive the OLEDs which, in turn, allows the OLED to operate in a more stable domain, thereby increasing its lifetime.
It is further known in the art that the human eye is less sensitive to spatial detail in low luminance colors than in high luminance colors. For this reason, displays, having a single layer of light emission, have been suggested which include fewer blue and/or red light-emitting elements than green. For example, U.S. Pat. No. 5,113,274 issued to Takahashi et al. and entitled “Matrix-type color liquid crystal display device” provides for a liquid crystal display that consists of a matrix of light-emitting elements wherein fewer red and blue light emitting elements are employed than green light-emitting elements. However, this principle has not been applied display structures having more than one layer of independently addressable light emitting elements.
There is a need, therefore, for an OLED device structure that reduces the tolerances for patterning different OLED materials, has a high efficiency, has the capability for forming a high resolution display device, and that is simpler to construct than a device having three-or-more layers of independently addressable light emitting elements.